The objective of this work has been to determine the uptake of radioactive-labeled serum albumin (RSA) and low density lipoprotein (LDL) by rabbit thoracic aorta in vivo and to analyze the data by two-compartmental mathematical model to calculate the rate constants of influx, Kin and efflux, Kout. The uptake of 125I-RSA and of 125I-LDL has been determined at different times after single injections of the markers. The uptake was higher at the arch than at the descending aorta. On the basis of the current results the Kin for LDL and RSA are estimated to be 0.13 and .056 micron 1/cm2-hr respectively. LDL labeled in protein and cholesterol moieties (with 125I and 14C respectively) is currently being used to study the dissociation between the protein and the lipid uptake. The role of endothelium was determined in several in situ experiments; in these, the chest cavity of the rabbit was opened under anaesthesia and the segment of aorta between the arch and diaphragm was isolated between two catheters. This aortic segment was then infused with labeled material in situ, washed, fixed with 4./. HCHO for 15 min and excised. The endothelium was scraped off, intima-media split from adventitia and counted. THe relative distribution of labeled material was 10:100:70 for endothelium, intima-media and adventitia respectively.